Sorry for no screenshot, I typically close my browser windows ASAP when these things pop up. It was an add with this URL (very long) that claimed my system had a Trojan, was booting something, offered a free scan. The usual. There is over a whole page of this URL in my history within the same minute.

That ad was right because those ads are virus, malware and stuff! Only believe warnings that are not on a web page but it's own program pop up in a window entirely separate from every browser and you bought it from a brick and mortar store.